MAAS gives you different metadata_url depending on where you access /MAAS/metadata/latest/enlist-preseed/?op=get_enlist_preseed from. Since this was the last URL I could find in any access logs I replicated the request from the same subnet and then instead of returning an IP like it did when I was testing in my browser from another subnet, it was returning the hostname of my MAAS server. If you put down any DNS configuration in your subnet that’s what the earliest boot-stages will use. In our case we haven’t forwarded our MAAS subdomain from our DNS servers to it yet (since we’re still in “roll-out phase” (call it testing it with a hammer phase if you will)).
So when you’re troubleshooting MAAS, make sure to be on the same subnet you’re trying to boot from if you want to see what data the server will give back to you.